40 



irradiated carbon tetrachloride. 



We will continue with the respiratory pigments and consider hemoglobin. 

 Irradiation of a dilute aqueous solution of hemoglobin with 20, 000 r causes the 

 following reactions to take place as observed spec trophotometrically: Oxidation 

 of the tryptophan groups of globin, as shown by a decrease in the absorption 

 spectrum at 2800 ^ ; an attack on the porphyrin, as shown by a decrease of the 

 Soret band; and oxidation of the Fe ++ porphyrin to Fe+++, as shown by an in- 

 crease in the absorption spectrum at 6300 9 In fact, the oxidation of oxyhemo- 

 globin to methemoglobin proceeds in linear relation to the X-ray exposure. 



We may conclude from all of these experiments that ionizing radiations 

 acting on aqueous oxygenated solutions show definite specificity in agreement 

 with the assumption that the effects are essentially those produced by the free 

 radicals, OH and O2H. It is essential to remember that observations from 

 studies using large doses of X-ray cannot be extrapolated to those with doses 

 used to irradiate living cells, which do not produce immediate death. Large 

 amounts of radiation produce effects qualitatively different from those obtained 

 from exposures in the thousands. Whether the effects of thousands of roentgens 

 can be extrapolated to effects produced by hundreds or tenths roentgens is not 

 known. 



The second conclusion I would like to make concerns the biological im- 

 portance of the induced oxidations, what I have called enhancement of radiation 

 action. These are reactions produced by the free radicals originating from the 

 primary oxidations due to OH and O^H radicals. The former are more stable 

 than water radicals, and, as a consequence, can diffuse more efficiently. The 

 cell has continuously a large number of oxidizable substances that, on x irradia- 

 tion, will form free radicals. These will act by themselves either as reducing 

 or oxidizing agents. 



Regarding the effect of radiations on nucleic acids, I would venture the 

 opinion that they are rather resistant because they are well-protected by other 

 groups in the vicinity. This opinion is in agreement with the observations of 

 cytologists. What is inhibited by small doses of X-rays is the synthesis of nu- 

 cleoproteins, as was found by Mitchell and by Hevesy. 



The work done with pure enzyme solutions cannot be extrapolated to the 

 cell. Irradiation of enzyme solutions was performed in a system where the en- 

 zyme was the only reactant. In the cell, there are hundreds of enzymes, pro- 

 teins, carbohydrates, fats, and electrolytes, all capable of reacting with the 

 free radicals. However, as a general rule, one may say that a system that is 

 stable when irradiated in aqueous solutions will be stable when so-treated in the 

 cell. 



I am, of course, disappointed to see that the -SH groups in the cell seem 

 to be more resistant than when they are irradiated in solution. However, I still 

 believe that the enzymes for nucleic acid synthesis and for protein synthesis are 

 -SH enzymes with freely-reacting -SH groups that are extremely sensitive to ox- 

 idizing agents. In this respect, I want to remind you of the experiments of Van 

 Heijningen (10), who found inhibition of -SH enzymes in the lens of X irradiated 

 rabbits, whereas enzymes possessing no essential -SH groups for activity were 

 not decreased. 



PATT: But this is somewhat removed in time from the initial event of 

 irradiation. Sulfhydryl inhibition, in this case, occurs days after irradiation and 

 therefore, probably does not represent a direct effect of oxidizing agents formed 



